1. Field of the Invention
The present invention relates to methods and apparatus for detecting parameters of a liquid crystal display for use, for example, in a liquid crystal display unit. More particularly, the present invention relates to methods and apparatus for measuring the thickness of a liquid crystal layer and the twist angle of liquid crystal molecules within the liquid crystal layer.
2. Description of the Related Art
Liquid crystal displays (LCDs) having liquid crystal molecules oriented parallel to the surface of two substrates of the display, wherein the direction of orientation of the liquid crystal molecules are gradually twisted or rotated between the two substrates, are well known. One type of such liquid crystal displays is known as a twisted nematic liquid crystal display (hereinafter referred to as "TN display"), in which the direction of orientation is twisted 90 degrees from the front substrate to the back substrate. Another known twisted nematic liquid crystal display is a super twisted nematic liquid crystal display (hereinafter referred to as "STN display"), in which the direction of orientation of the liquid crystal molecules is twisted between 180 to 270 degrees. These liquid crystal displays are used widely in various liquid crystal display units, such as display units for word processors, personal computers and televisions.
For such liquid crystal display units, the image quality directly relates to the distance between the two substrates, and in particular, to the uniformity of the thickness of the liquid crystal layer. The thickness determines the distribution of the twist angle of the liquid crystal molecules and is directly related to the contrast properties of the LCD. The uniformity of cell thickness and twist angle distribution throughout the entire panel therefore determines the display performance of LCDs. In addition, the anchoring energy (i.e., the orientation restricting force in the azimuth angle direction on each substrate surface of the liquid crystal display), which is another important parameter of liquid crystal displays, can be determined from the thickness of the liquid crystal layer and the twist angle of the liquid crystal molecules between the two substrates. Therefore, the quick and accurate determination of the thickness of the liquid crystal layer and the twist angle of the liquid crystal molecules are long-felt needs for both practical manufacturing processes and fundamental physical studies of liquid crystals.
The following methods have been developed to determine such liquid crystal display parameters: (1) measuring the electrostatic capacity and optical phase difference of a liquid crystal display to determine the thickness of the liquid crystal layer; (2) measuring light interference and electrostatic capacity of an empty display before the liquid crystal molecules are sealed between the substrates, to thereby determine the thickness of the liquid crystal layer; (3) measuring the distance between the two substrates using a three-dimensional shape measuring method, to thereby determine the thickness of the liquid crystal layer; (4) disposing a liquid crystal display between two polarizing plates and adjusting the optical path difference using a photo-elastic modulation element to perform curve fitting of the transmitted light intensity characteristics and determine the twist angle of the liquid crystal molecules; and (5) disposing a liquid crystal display between two polarizing plates and rotating one or both of the liquid crystal display and the polarizing plates to find an angle at which the maximum or minimum transmitted light intensity is transmitted, to thereby determine the thickness of the liquid crystal layer and the twist angle of the liquid crystal molecules in the layer using the Jones Matrix Display. These methods are described, for example, in "J. Appl. Phys." (Vol. 69, pages 1304-1309, 1991), "Jpn. J. Appl. Phys." (Vol.33, pages L434-L436, 1994), "Jpn. J. Appl. Phys." (Vol.33, pages L1242-L1244, 1994), "Jpn. J. Appl. Phys." (Vol.35, pages 4434-4437, 1996), "Lecture Manuscripts for the 22.sup.nd Panel Discussion for Liquid Crystal" (pages 139-140).
These methods may be classified into two types: extinction measurement methods and optoelectrical methods. The extinction measurement methods require some complicated rotation operations of the liquid crystal cell and/or polarizers for determine the angles of maximum and minimum transmissions. The optoelectrical measurement methods involve measuring optical transmissions at two different frequencies while rotating the analyzer and fitting the experimental curve using the Jones matrix method. Further, these methods are basically suited to measuring cell thickness and twist angle at only one probing point. In order to map a two-dimensional cell, the LCD must be mechanically moved from point to point. Thus, measurements of the cell thickness and twist angle of an entire display can not be performed quickly and with high accuracy using these methods.